Earlier quoted context omitted.
Here's the ACLU's page on this, with a map of the 100 mile "constitution-free zone": https://www.aclu.org/know-your-rights-governments-100-mile-b... Note that this zone includes New York, Los Angeles, Chicago, Houston, Seattle, etc, etc, and hence a really sizable portion of the US population.
Also note that that image is incomplete: international airports count as border crossings for CBP's purposes, so any city with one and the surrounding area in a 100 mile radius should be added. Edit: About like this. Quick & dirty edit of the ACLU's .jpg, so the quality is poor: http://imgur.com/a/DqDeQ
US Census population density map by county (pdf): http://www2.census.gov/geo/pdfs/maps-data/maps/thematic/us_p...